It will simply be a war of attrition as I stated. Those with better resources and manpower will win. Any gambling man would place a bet on AS. It would be like the Eastern Front where the Soviet Union fought against the main bulk of Nazi Germany, a grinding war of attrition.
The core group of fighters you mention will swell with more brainwashed recruits. Like I said, they just have to play a waiting game and train more kids forcefully taken from their families. Your million against millions of their soldiers of heaven? I know who would win. How many of your folk would commit suicidal human wave attacks? How many Reer Baadiyo would you able to raise considering how urbanised your clan is? Reer Magaal do not have the stomach for a fight as much as the hardier Reer Baadiyo. Without IC funding, would your diaspora resources match the revenue stream of the efficient AS bureaucracy and it's access to more finance generating resources in the richer South? How long can Qaadhan sustain a war?
Even if it starts off as a stalemate, they would grind down the civilian population with terror attacks against isolated nomads and cut off your supply of goods from the South such as fruits and vegetables. Will you be able to defend every isolated village along a hypothetical border? Sure there is Ethiopia as an alternative source of goods and imports etc. but that will only drive up the cost of living further impoverishing a society at war thus weakening their resolve.
Nonetheless, this is all hypothetical, it will never get to that point imo. If the IC abandon Somalia; Ethiopia and Kenya will step in to support their allied states. They will be guaranteed supplies and the Ethiopians, in particular, will probably throw in a few mechanised divisions.
I don't see how Alshabab, even with all of the manpower and resources of Southern Somalia, could hold Mudug let alone the rest of Puntland. Alshabab could conquer everything from Galkacyo to Bosaso, but they would not be able to remain there longer than 30 days. They can throw a punch but they would run out of breath long before the population of Puntland gives in.
The Taliban with all of the resources of Afghanistan could not defeat the Northern Alliance, Puntland has much more of Somalia's resources than the Northern Alliance had of Afghanistan's resources.
Alshabab would be risking complete defeat if they actually waged an all out conflict against Puntland.